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The rules forum gets a lot of questions on ready an action, and I've also noticed that many posts not actually asking about ready an action make false assumptions about how the mechanic works. So it seemed like an FAQ on the subject would, at the very least, save me a lot of retyping.

Where are the rules on ready an action?

Page 247 of the Rules Compendium. I imagine it's also in the DMG, HotFL, and HotFK. You'll also need to understand the rules on immediate actions, page 195. The section on readying actions has had some slight tweaks in the RC, so this is the preferable version to use.

In brief, how does ready an action work?

On your turn, you spend a standard action to ready an action. You then choose a target (if applicable), a triggering circumstance, and a specific action to ready. When/if the trigger action takes place, and it's legal for you to perform the action, you perform the action as an immediate reaction to the trigger action. Then you change your place in the initiative order so that you take your turn before the creature who triggered your readied action.

What if the triggering action never happens?

Then you wasted the standard action, and your initiative does not change. This is one of the risks of readying an action.

5. What is the difference between readying an action and delaying your turn?

Delaying your turn delays your entire turn until later in the round. Readying an action allows you to set up a specific action to happen later, contingent on a trigger condition. Both of them change your place in the initiative order.

Can I ready an action I can't legally perform at the time I ready it? For example, can I ready a move action when I'm immobilized?

Nothing in ready an action block states that the action has to be one you can perform at the time you ready it. Just be careful that you pick a legal trigger and target, see below.

Becuase some people seem to have problems with this: before you nerf this or cry that it's combat cheese, you need to consider that a character readying an action is taking a certain amount of risk. There is always the possibility that by the time the readied action is triggered, if the trigger happens at all, battlefield conditions may well have changed such that the readied action is impossible. Readying an action is not a guaranteed way of getting around conditions like immobilized, at best, they offer characters a reasonable shot.

Can I ready an action to trigger from a non-action trigger, like making a saving throw or beginning a turn?

It depends on how your DM defines "circumstance." Triggering can be very broad, your DM is probably going to have to make jugement calls on what he or she feels is a valid trigger.

Board consensus seems to be that a trigger should be an observable event. That is, it should be something characters in the game could plausibly perceive. That would rule out beginning or ending a turn, but might include succeeding on a saving throw (Your character no longer being on fire, for example, should certainly be obvious to the naked eye). A reminder: normally, saving throws are done at the end of your own turn, and thus, are not legal triggers because of the rules on the timing of immediate actions (see the entry below). Saving throws granted by powers when it isn't your turn are another story. It might also include conditions or beneficial effects expiring, depending on how "observable" your DM feels these events to be.

Again, this is board consensus, not RAW. The language about triggers is vague, I feel it's intentionally so to allow DMs and players some flexibility.

Can I use a readied action on my own turn?

No. Readied actions are immediate reactions, and thus follow all the rules for immediate actions, one of which is that they can't be used on your own turn.

Can I ready two or more actions at once?

If you can get more than one standard action to spend, sure. However, note that since you only get one immediate action per round, you'll only get to use, at best, one of those readied actions. Any others will be wasted.

Can I ready an action outside of combat?

The rules do not specifically forbid this, but the ready an action mechanic is very heavily dependent on other mechanics that only have meaning when combat is actually happening, such as actions and initiative order, so the intent seems to be that it should be used inside of combat only.

If multiple characters (or monsters) ready actions that use the same trigger, in what order do they resolve?

The timing of immediate actions that use the same trigger is not defined in the rules. The DM will need to make a ruling in situations where the order in which they resolve is important. A suggested solution is resolving in descending order by initiative bonus (which is how initiative ties are handled).

If your trigger comes up more than once, can you ignore it the first time and take it the second (or third, or fourth, or whateverth) time?

While the text of ready an action states that if you ignore the trigger, the action is lost, most of us find that to be a very draconian interpretation. We recommend that players be able to trigger a readied action from any trigger that happens before the beginning of their next turn before declaring the action lost.

How does readying a full-discipline action work?

Full discipline powers require different actions for each part of the power, so each part needs to be readied separately. If you wanted to ready both the attack technique and the movement portion, you'd have to somehow get two standard actions to spend (maybe via an action point). It is important to remember the rule about not being able to use any parts of an encounter full-discipline power more than once. If you use the movement portion of your full-discipline power on your turn, then you won't be able to ready it unless the power is an at-will.

Can I trigger readied actions off of free actions? Wouldn't that mean I could trigger my own readied actions, since I can take free actions when it isn't my turn?

*sigh* Technically, this is legal. It is probably also a place where DM discretion needs to be exercised.

the trigger has to be something that happens as the result of a move, free, immediate, standard, or minor action

That would eliminate a lot of perfectly natural readied actions to non-action events. Although it is strict RAW, I don't believe that is RAI. Still, I guess you could always take a free action (to say something) whenever you perceive a non-action event... thus you could ready your action to trigger on that free action. Silly, but within RAW.

readying an action requires you to pick a specific target, and in this scenario, there is no target to be chosen.

I would posit that readying an action to "shoot the first creature to come around that corner" is viable and RAI (even though it doesn't follow strict RAW).

10. Can I ready an action outside of combat?[/b]
The rules do not specifically forbid this

I would posit that readying an action outside of combat is not within RAI.

More or less agree myself, but this is the rules forum, and what the text is saying is very clear. As many, many other people have said before, RAI is at best a guess. I'd go so far as to put in something about the rule being bent often for cinematic effect.

I would like to point out it is not a rule that you can only ready an action in response to another creature's action.

"When you ready an action, you prepare to react to a creature’s action or an event." (bold mine)

Then the text box goes on to say :

"Choose trigger: Choose the action that will trigger your readied action. When that action occurs, you can use your readied action."

It uses this phraseology throughout the text block. Triggering off "an event" is never mentioned (and it would then have to define what "an event" is somehow). So which takes precedence, the text above or the stuff in the text block? I'm assuming the stuff in the text block are the actual rules, and the text above it is descriptive.

If it's not, then this starts to cause other problems, for example, in the non-block text above "opportunity attack," it says that "moving past" someone provokes an attack of opportunity. Is this a rule? Because I (and, it seems, most of the forums) are operating under the assumption that it's the "moving provokes" in the text block that's actually defining the mechanics side of this.

But the rules aren't perfectly written, and if a large number of players (possibly a majority) believe RAI is different than strict RAW, that should be noted in a FAQ. Example: by strict RAW, flying creatures can be attacked by pit traps... but we would not actually advise DM's to follow that in a FAQ.

btw: I completely agree with gwydion9 on #6. I'm unsure what the reasoning against it would be. legba11: do you have a link to a discussion on it?

which takes precedence, the text above or the stuff in the text block? I'm assuming the stuff in the text block are the actual rules, and the text above it is descriptive.

Boxed text (for brevity) sometimes phrases rules that if taken strictly literally can be overly limiting compared to the main text. I could research some more examples if desired. Boxed text does not necessarily cover every rule and intent, and afaik it has not been stated that boxed text has primacy.

Recognizing the RAW text of "When you ready an action, you prepare to react to a creature’s action or an event" seems suitable for a FAQ.

"Choose trigger: Choose the action that will trigger your readied action. When that action occurs, you can use your readied action."

It uses this phraseology throughout the text block. Triggering off "an event" is never mentioned (and it would then have to define what "an event" is somehow). So which takes precedence, the text above or the stuff in the text block? I'm assuming the stuff in the text block are the actual rules, and the text above it is descriptive.

If it's not, then this starts to cause other problems, for example, in the non-block text above "opportunity attack," it says that "moving past" someone provokes an attack of opportunity. Is this a rule? Because I (and, it seems, most of the forums) are operating under the assumption that it's the "moving provokes" in the text block that's actually defining the mechanics side of this.

I think you are being far too strict. Just because they didn't put "choose the action or event" in the choose trigger block doesn't mean you should ignore the previous information and common sense.

So we are supposed to ignore everything in the book unless it is in a tan box, if it's not in a tan box it's flavor text and should be ignored? I'll keep that in mind.

If you are so strict as to limit it entirely to creature's actions then players could never ready actions in response to environmental situations. That's why the book says "a creature's action or an event". Not to mention ignoring everything outside the tan boxes invalidates a good portion of the rules.

If you don't allow readied actions to be triggered by events then;

Players couldn't.....

ready an action to jump onto a swinging platform when it gets near.

ready an action to bull rush the enemy cultist into the magical tide when it rolls in.

ready an action to jump off the boat and onto the dock before it crashes into the rocks.

ready an action to catch someone if they fall.

ready an action to fire when the fog parts.

The non-boxed text is important and IS rules text. Take a look at Second Wind; "Unless otherwise noted in the statistics block of a monster or a nonplayer character, this action is available only to player characters". Are you telling me that's irrelevant flavor text we should ignore?

I think you are being far too strict. Just because they didn't put "choose the action or event" in the choose trigger block doesn't mean you should ignore the previous information and common sense.

It's more that plus the problem of how you then interpret what "an event" means. It seems as though if they had meant this, they would have gone on to define it with the other crunchy rules stuff. It's absence seems conspicuous.

So we are supposed to ignore everything in the book unless it is in a tan box, if it's not in a tan box it's flavor text and should be ignored? I'll keep that in mind.

I stopped reading right here. It's fine if you want to disagree, but if you want to be insulting, then *go away*, I have no interest in reading what you have to say. I've made every effort to provide substantive points and real discussion. I see you disagree and I'm willing to explore your point, but without the abuse.

It's more that plus the problem of how you then interpret what "an event" means. It seems as though if they had meant this, they would have gone on to define it with the other crunchy rules stuff.

I stopped reading right here. It's fine if you want to disagree, but if you want to be insulting, then *go away*, I have no interest in reading what you have to say. I've made every effort to provide substantive points and real discussion. I see you disagree and I'm willing to explore your point, but without the abuse.

I'm sorry, I'm not trying to be abusive or start a fight, I'm trying to point out that the text outside the box IS rules text.

You are specifically arguing that the text above the tan blocks is not rules text, and I am trying to point at that it is.

Second wind
"Unless otherwise noted in the statistics block of a monster or a nonplayer character, this action is available only to player characters"

I'm sorry, I'm not trying to be abusive or start a fight, I'm trying to point out that the text outside the box IS rules text.

Thank you, debate may now proceed.

You are specifically arguing that the text above the tan blocks is not rules text, and I am trying to point at that it is.

Actually, no, that's not exactly what I'm trying to say. If I had to state the argument, I'd say I think the boxed stuff *takes precedence* or is *more important*. That's why I brought up the stuff about the OA section, because I see the same thing happening there: the text above is being breezy and descriptive, leaving the job of tightening terms and mechanics to the stuff in the text box. *Moving past" someone is not actually what provokes OA, it's "leaving a square adjacent to them," which you would usually do when moving past them. "Moving past" is way too vague to use as a mechanic, just like "an event" is. Which is why that language gets "tied down" in the text box.

Second wind
"Unless otherwise noted in the statistics block of a monster or a nonplayer character, this action is available only to player characters"

Grab
"helpless allies are treated as objects"

True, and valid. But again, I'm not actually arguing that text box stuff isn't rules, or that it's not important.

Also, I *am* willing to note there's a debate in the FAQ itself. *points up*

*edit*

I went back and read some of your examples, and they are quite compelling. I may use one of two of them, as i'm still tinkering with the wording.

Actually, no, that's not exactly what I'm trying to say. If I had to state the argument, I'd say I think the boxed stuff *takes precedence* or is *more important*.

I guess we differ there in that I see "creature's action or an event" to be offering an exception or addition, rather than to be making a generalization that the box text will clear up.

Ie, why would they put "or an event" there if they were just going to immediately follow it with text that says only a creature's action can trigger it, ignore the phrase "or an event".

I could agree with you if the text above the block said "you prepare to react to something" or "you prepare to react to an event". Then when it uses action later on it could be locking down what that event or "something" could be.
But, the text says "you prepare to react to a creature's actionoran event". In my opinion that is very clearly telling us that in addition to creature's actions, you can ready in response to events. (and then they just don't mention events afterward for brevity)

Also, I *am* willing to note there's a debate in the FAQ itself. *points up*

Well, [i]technically[/i], you can set up a situation to react to anythign you want easily enough. Simply have someone else in your party use a Free [i]Action[/i] to say a triggering word and bingo, you have your triggering action.

Since that is indeed RAW, it's probably best to allow pretty much any action or event as a trigger to avoid such silliness. I think this is RAI and will play better than forcing the use of a "Free Action" (speak) from a party member as the trigger.

Ouch... With a narrow interpretation, you could disallow the tried and true rogue tactic of readying an action for an ally to move into flanking, since that's not the enemy action that triggering it...

Ouch... With a narrow interpretation, you could disallow the tried and true rogue tactic of readying an action for an ally to move into flanking, since that's not the enemy action that triggering it...

They can still delay their turn and take their entire turn after their ally has moved into a flanking position and taken their turn.

It might also be worth mentioning some of the "oddities" which arise in the course of using readied actions.

For example it is quite possible for characters to benefit multiple times from buffs or debuffs which are generally intended to function only once. A simple example

A C M are a set of three combatants where C is a cleric M is a monster and A is another PC. A readies a melee attack against M, triggered on C attacking M. C uses Righteous Brand on M, assigning the buff to A. A now interrupts C and makes an attack on M using the buff. Note that A's place in the turn order does not change. Next turn A can attack M again and still benefits from the buff since C's next turn has not yet ended.

This is actually a mild example. With the proper setup C can perpetuate the buff indefinitely at the cost of sacrificing its actions. This can also be generalized to other situations.

Readying can also be used to exploit the once per turn limit on immediate interrupts or once per enemy turn limit on opportunity actions.

For example a group of orcs may want to get past a fighter. Normally they would each move in their turn and the fighter would OA each one, stopping any that are hit. However if orcs A and B ready their move action and key it off of orc C then they will all technically be moving in orc C's turn and the fighter can only issue an OA against one of them. The other two orcs proceed past the fighter unimpeeded.

Similarly a CC can be avoided entirely by a monster simply by triggering its action off some action taken by the fighter, who cannot utilize an immediate interrupt in his own turn.

It is interesting to note that each of these examples utilizes triggering off an ally. The same tricks CAN be pulled triggering off enemy actions, but they will generally be less reliable and in some cases not possible. This may be one of the reasons why RAW appears to restrict readied actions to triggering off enemy actions only. It doesn't prevent exploitation, but it certainly does make it a riskier proposition.

It may be worth noting these issues with readying in a thorough FAQ, even though they really don't have answers (all being well within RAW).

It might also be worth mentioning some of the "oddities" which arise in the course of using readied actions.

For example it is quite possible for characters to benefit multiple times from buffs or debuffs which are generally intended to function only once. A simple example

A C M are a set of three combatants where C is a cleric M is a monster and A is another PC. A readies a melee attack against M, triggered on C attacking M. C uses Righteous Brand on M, assigning the buff to A. A now interrupts C and makes an attack on M using the buff.

Note that A's place in the turn order does not change. Next turn A can attack M again and still benefits from the buff since C's next turn has not yet ended.

RAW A moves in the initiative order to just before C, per the rule on page 291; which still allows this trick. As a DM I'd rule that A moves to just after C since it wasn't an interrupt, it was a reaction as A didn't go before C (if he did the brand wouldn't be in place for the first attack).

This is actually a mild example. With the proper setup C can perpetuate the buff indefinitely at the cost of sacrificing its actions. This can also be generalized to other situations.

You can't really avoid ending your turn.

For example a group of orcs may want to get past a fighter. Normally they would each move in their turn and the fighter would OA each one, stopping any that are hit. However if orcs A and B ready their move action and key it off of orc C then they will all technically be moving in orc C's turn and the fighter can only issue an OA against one of them. The other two orcs proceed past the fighter unimpeeded.

This I don't see as a problem; it's part of what readied actions are there for.

Similarly a CC can be avoided entirely by a monster simply by triggering its action off some action taken by the fighter, who cannot utilize an immediate interrupt in his own turn.

And if the fighter doesn't trigger the event the monster stands around doing absolutely nothing for the entire round. Then again you're the DM, you don't need rules tricks to screw your players.

RAW A moves in the initiative order to just before C, per the rule on page 291; which still allows this trick. As a DM I'd rule that A moves to just after C since it wasn't an interrupt, it was a reaction as A didn't go before C (if he did the brand wouldn't be in place for the first attack).

You can't really avoid ending your turn.

This I don't see as a problem; it's part of what readied actions are there for.

And if the fighter doesn't trigger the event the monster stands around doing absolutely nothing for the entire round. Then again you're the DM, you don't need rules tricks to screw your players.

Oh, I wasn't saying any of these things are PROBLEMS, they are just oddities of the readying system.

There are ways to pretty much perpetually delay the end of your turn though. It is a little tricky and involves having multiple enemies in the turn order, but you actually CAN do it. You can delay for up to as many turns as there are enemies in the turn order. It certainly wasn't an intentional part of the rules, but it GENERALLY isn't a huge problem since the best you can do is sacrifice any chance to use actions in order to extend a turn end effect, or avoid one. So far I haven't seen where anyone has managed to even house rule it away anyway without doing violence to the whole concept of readying.

I'm not sure where it is now, but there is a fairly recent thread someplace around here that explains the whole perpetual delay tactic. After looking at it I have to consider it conforms with RAW, but 99% of players would never even realize it can happen, on purpose. It is actually possible to do by accident though!

What happens if multiple people are readied off the same action? For example the whole party readies an attack on the closest enemy when the leader shouts charge ;).

In what order do they go when the leader shouts?

Also I don't see why you can't ready an action out of combat? Combat after all is nothing more then a really slowed down version of what's going on out of combat. 1 round is 6 seconds so why can't you assume you ready an action for a round it rolls back around to your init and then you ready it again? The whole point of init is mostly to determine who got the quickest draw/reaction. Because once init starts it basically a giant loop of each char taking their turn so who went first doesn't matter as much a few rounds into it. Player A goes after Monster B unless something changes init order like a readied action or delay turn.

So readying an action out of combat to get in the first attack is like setting up an ambush. But doesn't mean the players will get a surprise round. The monsters should get a perception check to see if they notice the players begin the assault. If the monsters fail perception check then players get surprise but if they succeed the players with the readied action simply get to go first with no surprise round. Which since init is actually more of a loop isn't not really that ground breaking any more then improved Init feat would be 'broken'. Since odds are players won't be able to pull off that kind of setup all the time or even on a potentially regular basis.

All that being said I also think the rules also say you must declare an intended target. So I don't think something like "The first enemy that comes around the corner." would be a valid target since it's not a specific figure and thus not a valid target. You might as well say "whatever enemy i feel like attacking". That's not a target but an enemy coming around the corner would be a potential trigger. This "attack the first enemy I see" is often the ready action out of combat that players try to do which I don't think is a valid ready action with RAW.

PHB1, pg. 291
Choose the specific action you are readying (what attack you plan to use, for example) as well as your intended target.

A "group" of potential targets is not an intended target which is why I think "Attack first enemy I see" is not a valid ready action.

So readying an action out of combat to get in the first attack is like setting up an ambush. But doesn't mean the players will get a surprise round. The monsters should get a perception check to see if they notice the players begin the assault. If the monsters fail perception check then players get surprise but if they succeed the players with the readied action simply get to go first with no surprise round. Which since init is actually more of a loop isn't not really that ground breaking any more then improved Init feat would be 'broken'. Since odds are players won't be able to pull off that kind of setup all the time or even on a potentially regular basis.

Except they will be able to do it every single time they come to a door, or a dozen other places. The surprise mechanism and initiative are THERE to determine who gets the jump on who. Characters are already immune to surprise in a 'door opening' situation. If the PCs surprise the monsters, then they can use their surprise actions to ready an attack. If they don't get surprise then quick reactions matter and the monsters may well come out of it pretty well.

There are other sorts of problems. Suppose the party opens a door, why would not the monsters have their bows all aimed at the door with actions readied? These things cut both ways.

The existing surprise mechanics provide a perfectly good way to use Ready WITHIN a combat situation to deal with setting up an ambush or a surprise attack. There just is not a need to give players another option that will generally just slow the game down by elaborating the door opening procedure even more than it is already. It won't add anything to the game IMHO.

It also starts to open up a whole other can of worms regarding other similar actions. Why wouldn't characters use their encounter long buff powers before entering combat for instance?

Abdul said it a lot better than I could. Readied actions outside combat = not a good idea.

Also, holy frijoles, I've been stickied!

I may add in something about multiple readied actions. My understanding of this situation is that the timing of multiple immediate triggered actions that all use the same trigger is not defined in the rules-the DM would have to make some sort of ruling as to what order they resolve in.

The general consensus (from prior threads) is to handle these in order of original initiative.

readying an action out of combat to get in the first attack is like setting up an ambush

As a general rule, I allow anyone that could've/would've readied an action before combat (were it allowed) to get an action on the surprise round.

A "group" of potential targets is not an intended target which is why I think "Attack first enemy I see" is not a valid ready action.

Not everyone would agree with that interpretation. Also, removing the ability to cover an area during combat (i.e. ready to "attack the first enemy I see coming around the corner") wouldn't seem to add fun or realism.

Not everyone would agree with that interpretation. Also, removing the ability to cover an area during combat (i.e. ready to "attack the first enemy I see coming around the corner") wouldn't seem to add fun or realism.

Not really. AoE powers target an area and you could have them trigger on when an enemy enters that area. The trigger is more open to general actions because it just says a triggering action. The target though says an intended target which in the case of an AoE power is the area not the monster.

After all watching an area and being ready to nuke it as soon as something enters it is one thing. It only takes a second to then finish off your power you were already primed to cast when compared to. Sitting there with bow drawn watching a whole area waiting for an enemy to show then when he does aiming at them and then firing.

This in my opinion is what Delay Turn is for. If you know the enemy is coming but don't wanna give up your turn to wait for a full init round you simply delay turn then when the monster comes around the corner and finishes it's turn you take your turn.

An intended target is not "I'll shoot this enemy, or this enemy, or this enemy. I'll let you know when the trigger happens." to leave the players options open. Those are the kind of things players have time to decide on their turns not in the brief moment it takes to do a readied action.

Except they will be able to do it every single time they come to a door, or a dozen other places. The surprise mechanism and initiative are THERE to determine who gets the jump on who. Characters are already immune to surprise in a 'door opening' situation. If the PCs surprise the monsters, then they can use their surprise actions to ready an attack. If they don't get surprise then quick reactions matter and the monsters may well come out of it pretty well.

There are more players then there are DMs at the table and the ultimate goal is to make sure the players have fun. If the players like feeling clever by readying a charge attack at every door then more power to them. It's about having fun. If the DM doesn't like this tactic well he's the DM! He can make it less advantageous. I had a DM were most dungeons have long hallways with few doors and those doors rarely had monsters right on the other side of them. They were usually opening into hallways that lead to a room with monsters.

Also think these are often old places made of stone. That means echos and doors that sqeek. So just because you open the door doesn't mean the enemy won't hear or see it, thus ruining the surprise factor.

There are other sorts of problems. Suppose the party opens a door, why would not the monsters have their bows all aimed at the door with actions readied? These things cut both ways.

Actually they could be readied. They usually aren't because it makes no sense from a roleplaying stand point. The players know they are about to spring on some bad guys, or at least assume they are. The monsters have been there for days, weeks, or etc. with nothing going on so their guard is down. This is why people always seem to get the drop on security guards. Because they have been their for so long they aren't really paying attention to what is going on anymore.

Now as for why they could be readied is simple. The might of setup an alarm trap for intruders. People often assume a trap's reaction has to be visible or have an immediate determent to the player. A simple pressure plate that activates a small bell in the room the bad guys are in when someone comes down the hall would suffice. The monsters hear it go off but because it's at the end of the hall behind a closed door the players don't. So the monsters take up positions to defend knowing someone is coming. And thanks to 4e new passive perception checks there is no need to roll anything to notify the players their was some trap to be noticed. If it is high enough though they can of course be alerted to the fact the pressure plate is there and thus avoid it.

The existing surprise mechanics provide a perfectly good way to use Ready WITHIN a combat situation to deal with setting up an ambush or a surprise attack. There just is not a need to give players another option that will generally just slow the game down by elaborating the door opening procedure even more than it is already. It won't add anything to the game IMHO.

I don't really see how it would slow down things that much as the player simply states the readied action they are going to do. They can't do a full round worth of action the most they could do is launch an attack or declare they are running into the room.

Also if you think like I do that when it says you need to pick an "intended target" this means a specific target. Not a general whatever enemy happens to be in range. Then it's rather difficult for the players to ready an attack since they have no intended targets. It's been established you can choose an invalid target in hopes that it will be valid when the trigger happens but that is still choosing A target. So the players can't really do much since they don't have any targets to choose from, even invalid ones, since they don't know what's in the room. Thus readying attack options is no really possible.

Now again things like AoE could work here I think. Because the player can declare they will say launch a fire ball 5 squares into the room when whoever opens the door. Since it has both a specific target and trigger then it's doable. And I think it's perfectly acceptable that someone would open the door and simply launch a fireball into the room to catch the people off guard. I don't think it's wise because they might not be hostile or if the creatures are clustered close to that spot. But it's certainly a valid ambush tactic.

It also starts to open up a whole other can of worms regarding other similar actions. Why wouldn't characters use their encounter long buff powers before entering combat for instance?

?? Umm how does it open up a new can of worms?

Encounter long actions could be used right before combat begins even without ready actions. It is been stated very clearly that you can use your powers at any time. Powers that last for a whole encounter have been stated that it is just a general time of about 5 mins. I have played in games where we were curtain the big boss was in the next room so everyone cast their encounter long buffs that weren't also attack based right before we went into the room.

Encounter is just a general term it doesn't mean that the second there is no hostile enemy standing there is ends. And it has been stated in examples from WoTC that an encounter long power could last longer then one encounter if you rush to the next fight and don't take a short rest. But you don't get the benefits of a short rest. People seem to assume the short rest is automatic and the second a battle finishes they get everything back and effects end.

But don't take my word for it.

PHB pg 58
If you use a power out of combat it last for 5 min unless otherwise noted.
PHB pg 278
Until the End of the Encounter: The effect ends when you take a rest (short or extended) or after 5 minutes.

So you see they can already use powers that last the whole encounter right before they go into a fight. And this is actually a very sound tactic. My groups often would "buff up" before a fight in 3x so it's not something new to 4e.

The real trick is knowing what your gonna be facing before you open that door. If it's a bunch of weak kobolds you wasted your dailies, since that's what most encounter long buffs are.